Display devices, such as desk-top computer screens and direct view and projection television sets, include electron excited cathodoluminescent display devices such as cathode ray tubes. Cathode ray tubes (CRTs) function as a result of a scanning electron beam from an electron gun impinging on phosphors on a relatively distant glass screen. The phosphors absorb the energy from the electron beam and subsequently emit a portion of the energy, which is typically in the visible region of the electromagnetic spectrum. This visible emission is then transmitted through the glass screen to the viewer. Other display devices, such as field emission displays for use in flat panel display screens, which include cold cathode emission devices, and vacuum fluorescent displays for use in handheld calculators, which include hot cathode emission devices, also function as a result of electrons exciting phosphors on a screen.
Phosphors are inorganic or organic luminescent materials that may include "activator" atoms that can modify the emitted radiation, such that the emission is in the visible region, as well as modify the emission intensity and the persistence of the image. Phosphors should preferably be capable of maintaining luminescence (e.g., fluorescence) under excitation for a relatively long period of time (e.g., about 5 milliseconds) to provide superior image reproduction. Also, in general, it is desirable to produce highly pure phosphors to increase absorption of the available excitation energy by the activator that emits the required radiation, rather than being consumed by other impurities or "killer" centers, which would result in lower luminescence and lower efficiency. Therefore, the quality of the deposited phosphor is an important parameter.
Typically, in field emission displays, powder electroluminescent cells, and other electroluminescent articles, the phosphor is deposited on an insulating substrate coated with a transparent, conductive material such as of indium tin oxide (ITO). A method for producing deposits of phosphors (typically, separate tracks of red, green, and blue phosphors) is electrophoresis (i.e., electrophoretic deposition). In electrophoresis, phosphor particles are deposited from a suspension under the action of an electric field. The suspension typically includes a nonaqueous liquid, such as an alcohol, and an electrolyte, such as a salt of yttrium, aluminum, lanthanum, or thorium. Such metal salts make it possible to electrically charge the phosphor particles and serve as a binder making it possible to obtain a highly adhesive coating. The part coated can serve either as the anode (anaphoresis) or cathode (cataphoresis).
Certain electrophoretic deposition methods result in nonreproducible and inhomogeneous phosphor layers. Others result in the underlying coating (e.g., indium tin oxide) losing its transparency or otherwise being deteriorated as by discoloration and reduced conductivity. U.S. Pat. No. 5,536,383 (Van Danh et al.) discloses a method that avoids these problems; however, this method uses a suspension that includes an aliphatic alcohol (except methanol), a powdered luminescent material, a metal salt, as well as other organic materials such as nitromethane and a vegetable protein. The metal salts can be salts of rare earth and alkaline earth metals. They can also be thorium, aluminum and/or cobalt salts. Preferably, they are hydrated nitrate salts of Mg, La, Al, Th, and Co.
There is a need for other electrophoretic deposition methods that are simpler and substantially eliminate the deterioration (e.g., discoloration and reduced conductivity) of the transparent, conductive coating.